Catalysts for the production of ethylene oxide from ethylene and molecular oxygen are generally supported silver catalysts. Such catalysts are typically promoted with alkali metals. The use of small amounts of the alkali metals potassium, rubidium and cesium were noted as useful promoters in supported silver catalysts in U.S. Pat. No. 3,962,136, issued Jun. 8, 1976, and U.S. Pat. No. 4,010,115, issued Mar. 1, 1977. The use of other co-promoters, such as rhenium, or rhenium along with sulfur, molybdenum, tungsten and chromium is disclosed in U.S. Pat. No. 4,766,105, issued Aug. 23, 1988, and U.S. Pat. No. 4,808,738, issued Feb. 28, 1989. U.S. Pat. No. 4,908,343, issued Mar. 13, 1990, discloses a supported silver catalyst containing a mixture of a cesium salt and one or more alkali metal and alkaline earth metal salts.
U.S. Pat. No. 3,844,981 issued Oct. 29, 1974, U.S. Pat. No. 3,962,285 issued Jun. 8, 1976 and British Patent No. 1,325,715, published Aug. 8, 1973, disclose the use of silver-rhenium ethylene oxide catalysts. In these patents a high surface area silver derivative such as silver oxide is impregnated with a rhenium solution and subsequently reduced to provide metallic rhenium alloyed with the silver. The '285 patent discloses the use of KOH to precipitate Ag.sub.2 O from AgNO.sub.3. There is no disclosure in the patents of the use of suitable inert supports such as porous refractory supports. U.S. Pat. No. 4,548,921, issued Oct. 22, 1985, discloses the use of rhenium in silver-supported ethylene oxide catalysts. In this reference, the rhenium is first placed on the support in the form of finely divided metal particles and the silver is subsequently deposited on the outer surface of the particles. U.S. Pat. No. 3,316,279, issued Apr. 25, 1967, discloses the use of rhenium compounds, particularly ammonium and alkali metal perrhenates for the oxidation of olefins to olefin oxides. In this reference, however, the rhenium compounds are used, unsupported, along with a reaction modifier (cyanides, pyridines or quinolines) in a liquid phase reaction. U.S. Pat. No. 3,972,829, issued Aug. 3, 1976, discloses a method for distributing catalytically active metallic components on supports using an impregnating solution of catalyst precursor compound and an organic thioacid or a mercaptocarboxylic acid. Catalytically active metals include metals of Groups IVA, IB, VIB, VIIB and VIII, including rhenium and which may be in either the oxidized or reduced state. U.S. Pat. No. 4,459,372, issued Jul. 10, 1984, discloses the use of rhenium metal in combination with a surface metallated (using Ti, Zr, Hf, V, Sb, Pb, Ta, Nb, Ge and/or Si) alumina or silica. U.S. Pat. No. 4,005,049, issued Jan. 25, 1977, teaches the preparation of a silver/transition metal catalyst useful in oxidation reactions. In this instance, the silver serves as both a catalyst and a support for the transition metal co-catalyst. In U.S. Pat. No. 4,536,482, issued Aug. 20, 1985, catalytically active metals such as Ag and Re are co-sputtered along with a co-sputtered support material on a particular support. U.S. Pat. No. 4,257,976, issued Mar. 24, 1981, discloses a catalyst combination of reduced silver, a carbonate of a rare earth metal and yttrium, a salt of an alkali or alkaline earth metal and a catalyst carrier. U.S. Pat. No. 4,766,105, issued Aug. 23, 1988, discloses adding a rhenium co-promoter selected from sulfur, molybdenum, chromium, tungsten and mixtures thereof to an alkali metal/rhenium doped supported silver catalyst. However, none of these references discloses the use of phosphorus and/or boron rhenium co-promoters on a silver-based, alkali metal/rhenium-doped supported catalyst and the resulting improvements in initial selectivity of the catalysts.